Java程序辅导

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7
JavaScript: Control 
Statements I

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7.1 Introduction
• The techniques you will learn here are applicable 
to most high-level languages, including 
JavaScript
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7.2 Algorithms
• Any computable problem can be solved by 
executing a series of actions in a specific order
• A procedure for solving a problem in terms of the 
actions to execute and the order in which the 
actions are to execute is called an algorithm
• Specifying the order in which statements are to be 
executed in a computer program is called 
program control

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7.3 Pseudocode
• Pseudocode 
– An artificial and informal language that helps 
programmers develop algorithms
– Carefully prepared pseudocode may be converted easily to 
a corresponding JavaScript program
– Normally describes only executable statements—the 
actions that are performed when the program is converted 
from pseudocode to JavaScript and executed
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Software Engineering Observation 7.1
Pseudocode is often used to “think out” a 
program during the program-design process. 
Then the pseudocode program is converted to 
a programming language such as JavaScript.

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7.4 Control Structures 
• Sequential execution
– Execute statements in the order they appear in the code
• Transfer of control
– Changing the order in which statements execute
• All programs can be written in terms of only three control 
structures
– sequence 
– selection 
– repetition
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7.4 Control Structures (Cont.) 
• Flowchart 
– A graphical representation of an algorithm or of a portion 
of an algorithm
– Drawn using certain special-purpose symbols, such as 
rectangles, diamonds, ovals and small circles
– Symbols are connected by arrows called flowlines, which 
indicate the order in which the actions of the algorithm 
execute

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7.4 Control Structures (Cont.) 
• In a flowchart that represents a complete algorithm, an 
oval symbol containing the word “Begin” is the first 
symbol used; an oval symbol containing the word “End” 
indicates where the algorithm ends. 
• In a flowchart that shows only a portion of an algorithm, 
the oval symbols are omitted in favor of using small circle 
symbols, also called connector symbols.
• Perhaps the most important flowcharting symbol is the 
diamond symbol, also called the decision symbol, which 
indicates that a decision is to be made. 
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Fig. 7.1 | Flowcharting JavaScript’s sequence structure.
Indicates a portion of an algorithm

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7.4 Control Structures (Cont.) 
• JavaScript provides three selection structures.
– The if statement either performs (selects) an action if a 
condition is true or skips the action if the condition is false.
• Called a single-selection structure because it selects or ignores a 
single action or group of actions. 
– The if…else statement performs an action if a condition is true 
and performs a different action if the condition is false. 
• Double-selection structure because it selects between two different 
actions or group of actions.
– The switch statement performs one of many different actions, 
depending on the value of an expression.
• Multiple-selection structure because it selects among many different 
actions or groups of actions.
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7.4 Control Structures (Cont.) 
• JavaScript provides four repetition statements, 
namely, while, do…while, for and for…in.
• Keywords cannot be used as identifiers (e.g., for 
variable names).

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Common Programming Error 7.1
Using a keyword as an identifier is a syntax 
error.
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Fig. 7.2 | 
JavaScript 
keywords.
JavaScript 
keywords 
    
break case catch continue default 
delete do else false finally 
for function if in instanceof 
new null return switch this 
throw true try typeof var 
void while with   
Keywords that are reserved but not used by JavaScript 
abstract boolean byte char class 
const debugger double enum export 
extends final float goto implements 
import int interface long native 
package private protected public short 
static super synchronized throws transient 
volatile     
 

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7.4 Control Structures (Cont.) 
• Single-entry/single-exit control structures make it 
easy to build programs. 
• Control structures are attached to one another by 
connecting the exit point of one control structure 
to the entry point of the next. 
– Control-structure stacking. 
• There is only one other way control structures 
may be connected
– Control-structure nesting
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7.5 if Selection Statement
• The JavaScript interpreter ignores white-space characters
– blanks, tabs and newlines used for indentation and vertical spacing
• Programmers insert white-space characters to enhance 
program clarity
• A decision can be made on any expression that evaluates 
to a value of JavaScript’s boolean type (i.e., any 
expression that evaluates to true or false).
• The indentation convention you choose should be 
carefully applied throughout your programs
– It is difficult to read programs that do not use uniform spacing 
conventions

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Good Programming Practice 7.1
Consistently applying reasonable indentation 
conventions throughout your programs 
improves program readability. We suggest a 
fixed-size tab of about 1/4 inch or three spaces 
per indent.
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Fig. 7.3 | Flowcharting the single-selection if statement.
If grade >= 60, execute 
this statement
Otherwise

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Software Engineering Observation 7.2
In JavaScript, any nonzero numeric value in a 
condition evaluates to true, and 0 evaluates to 
false. For strings, any string containing one 
or more characters evaluates to true, and the 
empty string (the string containing no 
characters, represented as "") evaluates to 
false. Also, a variable that has been declared 
with var but has not been assigned a value 
evaluates to false.
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7.6 if...else Selection Statement
• Allows the programmer to specify that different 
actions should be performed when the condition 
is true and when the condition is false.

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Good Programming Practice 7.2
Indent both body statements of an if...else
statement. 
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7.6 if...else Selection Statement 
(Cont.)
• Conditional operator (?:) 
– Closely related to the if…else statement
– JavaScript’s only ternary operator—it takes three 
operands
– The operands together with the ?: operator form a 
conditional expression
– The first operand is a boolean expression
– The second is the value for the conditional expression if the 
boolean expression evaluates to true
– Third is the value for the conditional expression if the 
boolean expression evaluates to false

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Fig. 7.4 | Flowcharting the double-selection if else statement.
If the condition is met…
This is 
executed
Otherwise…
This is 
executed
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7.6 if...else Selection Statement 
(Cont.)
• Nested if…else statements 
– Test for multiple cases by placing if…else statements inside other 
if…else structures
• The JavaScript interpreter always associates an else
with the previous if, unless told to do otherwise by the 
placement of braces ({})
• The if selection statement expects only one statement in 
its body
– To include several statements, enclose the statements in braces ({ and })
– A set of statements contained within a pair of braces is called a block

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Good Programming Practice 7.3
If there are several levels of indentation, each 
level should be indented the same additional 
amount of space. 
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Software Engineering Observation 7.3
A block can be placed anywhere in a program 
that a single statement can be placed. 

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Software Engineering Observation 7.4
Unlike individual statements, a block does not 
end with a semicolon. However, each 
statement within the braces of a block should 
end with a semicolon.
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Common Programming Error 7.2
Forgetting one or both of the braces that 
delimit a block can lead to syntax errors or 
logic errors.

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7.6 if...else Selection Statement 
(Cont.)
• A logic error has its effect at execution time. 
• A fatal logic error causes a program to fail and 
terminate prematurely. 
• A nonfatal logic error allows a program to 
continue executing, but the program produces 
incorrect results.
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Good Programming Practice 7.4
Some programmers prefer to type the 
beginning and ending braces of blocks before 
typing the individual statements within the 
braces. This helps avoid omitting one or both 
of the braces.

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Software Engineering Observation 7.5
Just as a block can be placed anywhere a 
single statement can be placed, it is also 
possible to have no statement at all (the empty 
statement) in such places. The empty 
statement is represented by placing a 
semicolon (;) where a statement would 
normally be.
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Common Programming Error 7.3
Placing a semicolon after the condition in an 
if structure leads to a logic error in single-
selection if structures and a syntax error in 
double-selection if structures (if the if part 
contains a nonempty body statement). 

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7.7 while Repetition Statement
•while
– Allows the programmer to specify that an action is to be 
repeated while some condition remains true
– The body of a loop may be a single statement or a block 
– Eventually, the condition becomes false and repetition 
terminates
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Common Programming Error 7.4
If the body of a while statement never causes 
the while statement’s condition to become 
true, a  logic error occurs. Normally, such a 
repetition structure will never terminate—an 
error called an infinite loop. Both Internet 
Explorer and Firefox show a dialog allowing 
the user to terminate a script that contains an 
infinite loop.

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Common Programming Error 7.5
Remember that JavaScript is a case-sensitive 
language. In code, spelling the keyword while
with an uppercase W, as in While, is a syntax 
error. All of JavaScript’s reserved keywords, 
such as while , if and else , contain only 
lowercase letters.
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Fig. 7.5 | Flowcharting the while repetition statement.
If this statement 
is true…
Execute this statement…
And evaluate if this 
statement is still true
Break out of the cycle 
when the statement is 
false

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7.8 Formulating Algorithms: Counter-
Controlled Repetition
• Counter-controlled repetition 
– Often called definite repetition, because the number of 
repetitions is known before the loop begins executing
• A total is a variable in which a script accumulates 
the sum of a series of values
– Variables that store totals should normally be initialized to 
zero before they are used in a program
• A counter is a variable a script uses to count—
typically in a repetition statement
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7.8 Formulating Algorithms: Counter-
Controlled Repetition
• Uninitialized variables used in mathematical 
calculations result in logic errors and produce the 
value NaN (not a number)
• JavaScript represents all numbers as floating-
point numbers in memory
• Floating-point numbers often develop through 
division
• The computer allocates only a fixed amount of 
space to hold such a value, so the stored floating-
point value can only be an approximation

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Fig. 7.6 | Pseudocode algorithm that uses counter-controlled repetition to solve 
the class-average problem.
Set total to zero
Set grade counter to one
While grade counter is less than or 
equal to ten
Input the next grade
Add the grade into the total
Add one to the grade counter
Set the class average to the total 
divided by ten
Print the class av rage
20

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Inc.  All rights reserved.
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Fig. 7.7 | 
Counter-
controlled 
repetition to 
calculate a class 
average (Part 1 
of 3).
 1  
 2  
 4  
 5  
 6  
 7  
 8     
 9          
10        
47        
48     
49       
Click Refresh (or Reload) to run the script again
 
50     
51  
 
Increments 
gradeCounter by 1 
after each iteration of the 
loop
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Fig. 7.7 | Counter-controlled repetition to calculate a class average (Part 3 of 3).

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Common Programming Error 7.6
Not initializing a variable that will be used in 
a calculation results in a logic error that 
produces the value NaN—Not a Number. You 
must initialize the variable before it is used in 
a calculation.
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Software Engineering Observation 7.6
If the string passed to parseInt contains a 
floating-point numeric value, parseInt
simply truncates the floating-point part. For 
example, the string "27.95" results in the 
integer 27, and the string "–123.45" results in 
the integer –123. If the string passed to 
parseInt is not a numeric value, parseInt
returns NaN (not a number).

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44
Common Programming Error 7.7
Using floating-point numbers in a manner 
that assumes they are represented precisely 
can lead to incorrect results. Real numbers 
are represented only approximately by 
computers. For example, no fixed-size 
floating-point representation of π can ever be 
precise, because π is a transcendental number 
whose value cannot be expressed as digits in a 
finite amount of space.
23

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7.9 Formulating Algorithms: Sentinel-
Controlled Repetition
• Sentinel-controlled repetition
– Special value called a sentinel value (also called a signal 
value, a dummy value or a flag value) indicates the end of 
data entry 
– Often is called indefinite repetition, because the number of 
repetitions is not known in advance
• Choose a sentinel value that cannot be confused 
with an acceptable input value

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Common Programming Error 7.8
Choosing a sentinel value that is also a 
legitimate data value results in a logic error 
and may prevent a sentinel-controlled loop 
from terminating properly.
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7.9 Formulating Algorithms: Sentinel-
Controlled Repetition (Cont.)
• Top-down, stepwise refinement 
– A technique that is essential to the development of well-structured 
algorithms
– Approach begins with pseudocode of the top, the statement that conveys 
the program’s overall purpose
– Divide the top into a series of smaller tasks and list them in the order in 
which they need to be performed—the first refinement
– Second refinement commits to specific variables

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48
Software Engineering Observation 7.7
Each refinement, as well as the top itself, is a 
complete specification of the algorithm; only 
the level of detail varies.
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49
Error-Prevention Tip 7.1
When performing division by an expression 
whose value could be zero, explicitly test for 
this case, and handle it appropriately in your 
program (e.g., by printing an error message) 
rather than allowing the division by zero to 
occur.

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50
Good Programming Practice 7.5
Include completely blank lines in pseudocode 
programs to make the pseudocode more 
readable. The blank lines separate pseudocode 
control structures and separate the program 
phases. 
26

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51
Software Engineering Observation 7.8
Many algorithms can be divided logically into 
three phases: an initialization phase that 
initializes the program variables, a processing 
phase that inputs data values and adjusts 
program variables accordingly, and a 
termination phase that calculates and prints 
the results. 

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Fig. 7.8 | Sentinel-controlled repetition to solve the class-average problem.
Initialize total to zero
Initialize gradeCounter to zero
Input the first grade (possibly the sentinel)
While the user has not as yet entered the sentinel 
Add this grade into the running total
Add one to the grade counter
Input the next grade (possibly the sentinel)
If the counter is not equal to zero
Set the average to the total divided by the counter
Print the average
Else
Print “No grades were entered”
27

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53
Software Engineering Observation 7.9
The programmer terminates the top-down, 
stepwise refinement process after specifying 
the pseudocode algorithm in sufficient detail 
for the programmer to convert the 
pseudocode to a JavaScript program. Then, 
implementing the JavaScript program will 
normally be straightforward. 

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54
Good Programming Practice 7.6
When converting a pseudocode program to 
JavaScript, keep the pseudocode in the 
Java-Script program as comments.
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55
Software Engineering Observation 7.10
Experience has shown that the most difficult 
part of solving a problem on a computer is 
developing the algorithm for the solution. 
Once a correct algorithm is specified, the 
process of producing a working JavaScript 
program from the algorithm is normally 
straightforward.

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56
Software Engineering Observation 7.11
Many experienced programmers write 
programs without ever using program-
development tools like pseudocode. As they 
see it, their ultimate goal is to solve the 
problem on a computer, and writing 
pseudocode merely delays the production of 
final outputs. Although this approach may 
work for simple and familiar problems, it can 
lead to serious errors in large, complex 
projects. 
29

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57
7.9 Formulating Algorithms: Sentinel-
Controlled Repetition (Cont.)
• Control structures may be stacked on top of one 
another in sequence

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Inc.  All rights reserved.
58
Fig. 7.9 | 
Sentinel-
controlled 
repetition to 
calculate a class 
average (Part 1 
of 3).
 1  
 2  
 4  
 5  
 6  
 7  
 8     
 9        
10            
11        
60     
61     
62       
Click Refresh (or Reload) to run the script again
 
Increments 
gradeCounter by 1 
after each iteration of the 
loop
Begin new control 
structure
Execute if the condition in 
the if statement is not 
satisfied
Executes until gradeValue equals  
-1

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60
Fig. 7.9 | Sentinel-controlled repetition to calculate a class average (Part 3 of 3).
31

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61
Good Programming Practice 7.7
In a sentinel-controlled loop, the prompts 
requesting data entry should explicitly remind 
the user what the sentinel value is.

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62
Common Programming Error 7.9
Omitting the braces that delineate a block can 
lead to logic errors such as infinite loops.
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63
7.10 Formulating Algorithms: Nested 
Control Statements
• Control structures may be nested inside of one 
another

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64
Fig. 7.10 | Examination-results problem pseudocode.
Initialize passes to zero
Initialize failures to zero
Initialize student to one
While student counter is less than or equal to ten
Input the next exam result
If the student passed
Add one to passes
Else
Add one to failures
Add one to student counter
Print the number of passes
Print the number of failures
If more than eight students passed 
Print “Raise tuition” 
33

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Inc.  All rights reserved.
65
Fig. 7.11 | 
Examination-
results 
calculation (Part 
1 of 3).
 1  
 2  
 4  
 5  
 6  
 7  
 8     
 9        
10        
40     
41     
42       
Click Refresh (or Reload) to run the script again
 
43     
44  
 
Additional control structure
34

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67
Fig. 7.11 | Examination-results calculation (Part 3 of 3).

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68
Good Programming Practice 7.8
When inputting values from the user, validate 
the input to ensure that it is correct. If an 
input value is incorrect, prompt the user to 
input the value again.
35

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69
7.11 Assignment Operators
• JavaScript provides the arithmetic assignment 
operators +=, -=, *=, /= and %=, which 
abbreviate certain common types of expressions.

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70
Performance Tip 7.1
Programmers can write programs that 
execute a bit faster when the arithmetic 
assignment operators are used, because the 
variable on the left side of the assignment does 
not have to be evaluated twice. 
36

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71
Performance Tip 7.2
Many of the performance tips we mention in 
this text result in only nominal improvements, 
so the reader may be tempted to ignore them. 
Significant performance improvement often is 
realized when a supposedly nominal 
improvement is placed in a loop that may 
repeat a large number of times.

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72
Fig. 7.12 | 
Arithmetic 
assignment 
operators.
Assignment  
operator 
Initial value of 
variable 
Sample  
expression 
Explanation Assigns 
+= c = 3 c += 7 c = c + 7 10 to c 
-= d = 5 d -= 4 d = d - 4 1 to d 
*= e = 4 e *= 5  e = e * 5 20 to e 
/= f = 6 f /= 3 f = f / 3 2 to f 
%= g = 12 g %= 9 g = g % 9 3 to g 
 
37

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73
7.12 Increment and Decrement Operators
• The increment operator, ++, and the decrement 
operator, --, increment or decrement a variable 
by 1, respectively. 
• If the operator is prefixed to the variable, the 
variable is incremented or decremented by 1, 
then used in its expression. 
• If the operator is postfixed to the variable, the 
variable is used in its expression, then 
incremented or decremented by 1.

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74
Fig. 7.13 | 
Increment and 
decrement 
operators.
Operator Example Called Explanation 
++ ++a  preincrement Increment a by 1, then use the new value of 
a in the expression in which a resides. 
++ a++ postincrement Use the current value of a in the expression 
in which a resides, then increment a by 1. 
-- --b  predecrement Decrement b by 1, then use the new value of 
b in the expression in which b resides. 
-- b-- postdecrement Use the current value of b in the expression 
in which b resides, then decrement b by 1. 
 
38

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75
Error-Prevention Tip 7.2
The predecrement and postdecrement 
JavaScript operators cause the W3C XHTML 
Validator to incorrectly report errors. The 
validator attempts to interpret the decrement 
operator as part of an XHTML comment tag 
(). You can avoid this problem by 
using the subtraction assignment operator 
(-=) to subtract one from a variable. Note that 
the validator may report many more 
(nonexistent) errors once it improperly parses 
the decrement operator.

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Inc.  All rights reserved.
76
Fig. 7.14 | 
Preincrementing 
and 
postincrementing 
(Part 1 of 2).
 1  
 2  
 4  
 5  
 6  
 7  
 8     
 9        
10        
29     
30  
 
Prints value of c, then increments it
Increments c, then prints its value
39

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77
Fig. 7.14 | Preincrementing and postincrementing (Part 2 of 2).

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78
Good Programming Practice 7.9
For readability, unary operators should be 
placed next to their operands, with no 
intervening spaces.
40

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79
7.12 Increment and Decrement Operators 
(Cont.)
• When incrementing or decrementing a variable in 
a statement by itself, the preincrement and 
postincrement forms have the same effect, and 
the predecrement and postdecrement forms have 
the same effect
• When a variable appears in the context of a 
larger expression, preincrementing the variable 
and postincrementing the variable have different 
effects. Predecrementing and postdecrementing 
behave similarly.

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80
Common Programming Error 7.10
Attempting to use the increment or decrement 
operator on an expression other than a left-
hand-side expression—commonly called an 
lvalue—is a syntax error. A left-hand-side 
expression is a variable or expression that can 
appear on the left side of an assignment 
operation. For example, writing ++(x + 1) is 
a syntax error, because (x + 1) is not a left-
hand-side expression.
41

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81
Fig. 7.15 | 
Precedence and 
associativity of the 
operators discussed 
so far.
Operator Associativity Type 
++  --  right to left unary 
*   /   % left to right multiplicative 
+   - left to right additive 
<   <=  >   >= left to right relational 
==  != left to right equality 
?: right to left conditional 
=   +=  -=  *=  /=  %= right to left assignment